Ceramic Powder, Ceramic Layer and Layer System with Pyrochlore Phase and Oxides

ABSTRACT

A layer system is disclosed. The layer system includes a substrate and a ceramic outer layer. The ceramic outer ceramic layer is produced from a ceramic powder. The ceramic powder include a phrochlore phase according to the empirical formula A x B y O z  with x, y≈2, z≈7 and a secondary oxide C r O s  with r, s&gt;0.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of Ser. No. 12/151,423 filed May6, 2008, which claims the benefit of the provisional patent applicationfiled on May 7, 2007, and assigned application No. 60/928,086, and ofEuropean Patent Office application No. 07009129 EP filed May 7, 2007.All of the applications are incorporated by reference herein in theirentirety.

FIELD OF INVENTION

The invention relates to a ceramic powder, to a ceramic layer and to alayer system with pyrochlores and oxides.

BACKGROUND OF INVENTION

Such a layer system has a substrate comprising a metal alloy based onnickel or cobalt. Such products are used especially as a component of agas turbine, in particular as gas turbine blades or heat shields. Thecomponents are exposed to a hot gas flow of aggressive combustion gases.They must therefore be able to withstand heavy thermal loads. It isfurthermore necessary for these components to be oxidation- andcorrosion-resistant. Especially moving components, for example gasturbine blades, but also static components, are furthermore subject tomechanical requirements. The power and efficiency of a gas turbine, inwhich there are components exposable to hot gas, increase with a risingoperating temperature. In order to achieve a high efficiency and a highpower, those gas turbine components which are particularly exposed tohigh temperatures are coated with a ceramic material. This acts as athermal insulation layer between the hot gas flow and the metallicsubstrate.

The metallic base body is protected against the aggressive hot gas flowby coatings. In this context, modern components usually comprise aplurality of coatings which respectively fulfill specific functions. Thesystem is therefore a multilayer system.

Since the power and efficiency of gas turbines increase with a risingoperating temperature, attempts are continually being made to achieve ahigher performance of gas turbines by improving the coating system.

EP 0 944 746 B1 discloses the use of pyrochlores as a thermal insulationlayer. The use of a material as a thermal insulation layer, however,requires not only good thermal insulation properties but also goodmechanical properties and good bonding to the substrate.

EP 0 992 603 A1 discloses a thermal insulation layer system ofgadolinium oxide and zirconium oxide, which does not have a pyrochlorestructure.

SUMMARY OF INVENTION

It is therefore an object of the invention to provide a ceramic powder,a ceramic layer and a layer system having good thermal insulationproperties and good bonding to the substrate and therefore a longlifetime of the entire layer system.

The invention is based on the discovery that in order to achieve a longlifetime, the entire system must be considered as a whole and individuallayers or some layers together should not be considered and optimizedseparately from one another.

The object is achieved by a ceramic powder, a ceramic layer and a layersystem as claimed in independent claims.

The dependent claims describe further advantageous measures, which mayadvantageously be combined in any desired way.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a layer system according to the invention,

FIG. 2 shows a list of superalloys,

FIG. 3 shows a perspective view of a turbine blade,

FIG. 4 shows a perspective view of a combustion chamber,

FIG. 5 shows a gas turbine.

DETAILED DESCRIPTION OF INVENTION

The ceramic pyrochlore powder according to the invention of the generalformula A₂B₂O₇ has as a further constituent an oxide C_(r)O_(s) of ametal (O=oxygen; B=Hf, Zr, Ti, Sn; A=Gd, Sm, Nd, La, Y). The metalconstituent of the secondary oxide is denoted here by C.

The composition of the ceramic powder will also be explained by way ofexample with the aid of the composition of the ceramic layer 13 (FIG.1). In general, departures from the stoichiometry of the generalpyrochlore structure A₂B₂O₇ may always occur.

Pyrochlore structures in which A=gadolinium are preferably used, sincegood to very good thermal insulation properties are achieved in thiscase. Depending on the application, a hafnate or a zirconate will beused so that B=hafnium or zirconium.

Gadolinium hafnate or gadolinium zirconate will thus preferably be used.

Gadolinium hafnate as the powder comprises from 43 wt % to 50 wt %,preferably from 44.7 wt % to 47.7 wt % of gadolinium oxide, theremainder being hafnium oxide and optionally the secondary oxides,preferably only zirconium oxide, and the sintering aids.

Gadolinium zirconate as the powder comprises from 56 wt % to 63 wt %,preferably from 58 wt % to 61 wt % of gadolinium oxide, the remainderbeing zirconium oxide and optionally the secondary oxides, preferablyonly hafnium oxide, and sintering aids.

The ceramic layer 13 (FIG. 1) or the ceramic powder comprises apyrochlore phase of the general empirical formula A_(x)B_(y)O_(z) withx, y≈2, z≈7 and a secondary oxide C_(r)O_(s) with r, s>0. The secondaryoxide C_(r)O_(s) is in this case deliberately added to the powder and isthus significantly above the metrological detection limit of thesecondary oxide, i.e. it has at least two times the value of thedetection limit of the secondary oxide.

The secondary oxide has in particular a proportion of from 0.5 wt % to10 wt %, more particularly a proportion of from 1 wt % to 10 wt %. Themaximum proportion of the secondary oxide is preferably 8 wt %, inparticular at most 6 wt % and more particularly between 5 wt % and 7 wt%. The maximum proportion of the secondary oxide is likewise preferably3 wt %, in particular at most 2 wt % and more particularly between 1.5wt % and 2.5 wt %. In particular, the ceramic powder consists of atleast one pyrochlore phase and at least one secondary oxide.

For the secondary oxide, the oxide of B may be used (C=B) or not (C≠B).If C=B, then a high phase stability of the pyrochlore phase is ensured.If B≠C, however, then an increase in the mechanical strength isachieved.

Hafnium oxide or zirconium oxide therefore preferably used, since theyare particularly stable at high temperatures and they do not entaildiffusion and therefore phase modification of the pyrochlore structure.

The ceramic layer 13 or the ceramic powder preferably comprises only onepyrochlore phase, so that thermal stresses do not occur betweendifferent phases when used with strongly alternating temperatures.

A mixture of only two pyrochlore phases may likewise be used, i.e. forexample a powder mixture of Gd₂Zr₂O₇ and Gd₂Hf₂O₇, in order to combinethe improved thermal insulation properties of one pyrochlore phase withthe better thermal expansion coefficients of the other pyrochlore phase.This is the case, in particular, for gadolinium zirconate and gadoliniumhafnate.

The pyrochlore phase may likewise preferably be present as a mixedcrystal, so that good mixing will have already taken place here or phasestability is provided. This is the case, in particular, forGd₂(Hf_(x)Zr_(y))O₇ with x+y≈2.

The ceramic layer 13 or the ceramic powder preferably comprises only onesecondary oxide. The secondary oxide may constitute hafnium oxide orzirconium oxide. Zirconium oxide is preferably used when a hafnate isemployed as the pyrochlore phase. A zirconium oxide is preferably usedwhen a hafnate is employed for the pyrochlore phase.

Two secondary oxides, in particular hafnium oxide and zirconium oxide,may likewise be used so that the mechanical properties are improvedfurther.

The secondary oxides may in this case be present only as an oxide, sothat there is a secondary phase here which leads to mechanicalreinforcement, or they are present as a mixed crystal with one anotheror with the pyrochlore phase, so that the thermal conductivity can inthis way be reduced further by the stresses thereby generated in thelattice.

In order to draw advantages from both presentation types of thesecondary oxides, the secondary oxide or oxides may be present both asan oxide or as a mixed crystal in the pyrochlore phase.

Preferably, B≠C.

A pyrochlore powder of gadolinium zirconate, in particular Gd₂Zr₂O₇,thus comprises hafnium oxide in particular with a proportion of from 1.5wt % to 2.5 wt %, in particular 2 wt %.

Gadolinium hafnate, in particular Gd₂Hf₂O₇, preferably compriseszirconium oxide in particular with a proportion of from 5 wt % to 7 wt%, in particular up to 6 wt %.

The pyrochlore or pyrochlores preferably have the following optionalconstituents as sintering aids:

Up to 0.05 wt % silicon oxide,Up to 0.1 wt % calcium oxide,Up to 0.1 wt % magnesium oxide,Up to 0.1 wt % iron oxide,Up to 0.1 wt % aluminum oxide andUp to 0.08 wt % titanium oxide.

During the coating or during subsequent use at higher temperatures,these sintering aids lead to dense and stable layers.

No other sintering aids are preferably used.

FIG. 1 shows a layer system 1 according to the invention.

The layer system 1 comprises a metallic substrate 4 which, in particularfor components at high temperatures, consists of a nickel- orcobalt-based superalloy (FIG. 2). There is preferably a metallic bondinglayer 7 directly on the substrate 4, in particular of the NiCoCrAlXtype, which preferably comprises (11-13) wt % cobalt, (20-22) wt %chromium (10.5-11.5) wt % aluminum, (0.3-0.5) wt % yttrium, (1.5-2.5) wt% rhenium and the remainder nickel, or which preferably comprises(24-26) wt % cobalt, (16-18) wt % chromium (9.5-11) wt % aluminum,(0.3-0.5) wt % yttrium, (1-1.8) wt % rhenium and the remainder nickel,and in particular consists thereof.

An aluminum oxide layer is preferably formed already on this metallicbonding layer 7 before further ceramic layers are applied, or such analuminum oxide layer (TGO) is formed during operation.

There is preferably an inner ceramic layer 10, preferably a fully orpartially stabilized zirconium oxide layer, on the metallic bondinglayer 7 or on the aluminum oxide layer (not shown) or on the substrate4. Yttrium-stabilized zirconium oxide is preferably used, with 6 wt %-8wt % of yttrium preferably being employed. Calcium oxide, cerium oxideand/or hafnium oxide may likewise be used to stabilize zirconium oxide.The zirconium oxide is preferably applied as a plasma-sprayed layer,although it may also preferably be applied as a columnar structure bymeans of electron beam deposition (EBPVD).

An outer ceramic layer 13 of the ceramic powder is applied on thestabilized zirconium oxide layer 10 or on the metallic bonding layer 7or on the substrate. The layer 13 preferably constitutes the outermostlayer, which is exposed directly to the hot gas. The layer 13 consistsmainly of a pyrochlore phase, i.e. it comprises at least 90 wt % of thepyrochlore phase which preferably consists of either Gd₂Hf₂O₇ orGd₂Zr₂O₇.

The secondary oxides are distributed in the layer 13, preferablyhomogeneously distributed.

The layer thickness of the inner layer 10 is preferably between 10% and50% in particular between 10% and 40%, of the total layer thickness ofthe inner layer 10 plus the outer layer 13. The inner ceramic layer 10preferably has a thickness of from 100 μm to 200 μm, in particular 150μm±10%. The total layer thickness of the inner layer 10 plus the outerlayer 13 is preferably 300 μm or preferably 450 μm. The maximum totallayer thickness is advantageously 600 μm or preferably at most 800 μm.The layer thickness of the inner layer 10 is preferably between 10% and40% or between 10% and 30% of the total layer thickness. It is likewiseadvantageous for the layer thickness of the inner layer 10 to comprisefrom 10% to 20% of the total layer thickness. It is likewise preferablefor the layer thickness of the inner layer 10 to be between 20% and 50%or between 20% and 40% of the total layer thickness. Advantageousresults are likewise achieved if the contribution of the inner layer 10to the total layer thickness is between 20% and 30%. The layer thicknessof the inner layer 10 is preferably from 30% to 50% of the total layerthickness. It is likewise advantageous for the layer thickness of theinner layer 10 to comprise from 30% to 40% of the total layer thickness.It is likewise preferable for the layer thickness of the inner layer 10to be between 40% and 50% of the total layer thickness.

For short-term use at high temperatures of the layer system, the outerlayer 13 may preferably be configured to be thinner than the inner layer10, i.e. the layer thickness of the outer layer 13 is at most 40% of thetotal layer thickness of the inner layer 10 plus the outer layer 13.

The layer system preferably consists of the substrate 4, the metalliclayer 7, the inner ceramic layer 10 and the outer ceramic layer 13, andoptionally the TGO.

FIG. 3 shows a perspective view of a rotor blade 120 or guide vane 130of a turbomachine, which extends along a longitudinal axis 121.

The turbomachine may be a gas turbine of an aircraft or of a power plantfor electricity generation, a steam turbine or a compressor.

The blade 120, 130 comprises, successively along the longitudinal axis121, a fastening zone 400, a blade platform 403 adjacent thereto as wellas a blade surface 406. As a guide vane 130, the vane 130 may have afurther platform (not shown) at its vane tip 415.

A blade root 183 which is used to fasten the rotor blades 120, 130 on ashaft or a disk (not shown) is formed in the fastening zone 400. Theblade root 183 is configured, for example, as a hammerhead. Otherconfigurations as a firtree or dovetail root are possible. The blade120, 130 comprises a leading edge 409 and a trailing edge 412 for amedium which flows past the blade surface 406.

In conventional blades 120, 130, for example solid metallic materials,in particular superalloys, are used in all regions 400, 403, 406 of theblade 120, 130. Such superalloys are known for example from EP 1 204 776B1, EP 1 306 454, EP 1 319 729 A1, WO 99/67435 or WO 00/44949. Theblades 120, 130 may in this case be manufactured by a casting method,also by means of directional solidification, by a forging method, by amachining method or combinations thereof.

Workpieces with a monocrystalline structure or structures are used ascomponents for machines which are exposed to heavy mechanical, thermaland/or chemical loads during operation. Such monocrystalline workpiecesare manufactured, for example, by directional solidification from themelts. These are casting methods in which the liquid metal alloy issolidified to form a monocrystalline structure, i.e. to form themonocrystalline workpiece, or is directionally solidified. Dendriticcrystals are in this case aligned along the heat flux and faun either arod crystalline grain structure (columnar, i.e. grains which extend overthe entire length of the workpiece and in this case, according togeneral terminology usage, are referred to as directionally solidified)or a monocrystalline structure, i.e. the entire workpiece consists of asingle crystal. It is necessary to avoid the transition to globulitic(polycrystalline) solidification in these methods, since nondirectionalgrowth will necessarily form transverse and longitudinal grainboundaries which negate the beneficial properties of the directionallysolidified or monocrystalline component.

When directionally solidified structures are referred to in general,this is intended to mean both single crystals which have no grainboundaries or at most small-angle grain boundaries, and also rod crystalstructures which, although they do have grain boundaries extending inthe longitudinal direction, do not have any transverse grain boundaries.These latter crystalline structures are also referred to asdirectionally solidified structures.

Such methods are known from U.S. Pat. No. 6,024,792 and EP 0 892 090 A1.

The blades 120, 130 may likewise have coatings against corrosion oroxidation, for example (MCrAlX; M is at least one element from the groupion (Fe), cobalt (Co), nickel (Ni), X is an active element and standsfor yttrium (Y) and/or silicon and/or at least one rare earth element,or hafnium (Hf)). Such alloys are known from EP 0 486 489 B1, EP 0 786017 B1, EP 0 412 397 B1 or EP 1 306 454 A1.

On the MCrAlX layer, there may furthermore be a ceramic thermalinsulation layer 13 according to the invention. Rod-shaped grains areproduced in the thermal insulation layer by suitable coating methods,for example electron beam deposition (EB-PVD).

Refurbishment means that components 120, 130 may need to have protectivelayers taken off (for example by sandblasting) after their use. Thecorrosion and/or oxidation layers or products are then removed.Optionally, cracks in the component 120, 130 are also repaired. Thecomponent 120, 130 is then recoated and the component 120 is used again.

The blade 120, 130 may be designed to be a hollow or solid. If the blade120, 130 is intended to be cooled, it will be hollow and optionally alsocomprise film cooling holes 418 (indicated by dashes).

FIG. 4 shows a combustion chamber 110 of a gas turbine 100 (FIG. 5). Thecombustion chamber 110 is designed for example as a so-called ringcombustion chamber in which a multiplicity of burners 107, which produceflames 156 and are arranged in the circumferential direction around arotation axis 102, open into a common combustion chamber space 154. Tothis end, the combustion chamber 110 as a whole is designed as anannular structure which is positioned around the rotation axis 102.

In order to achieve a comparatively high efficiency, the combustionchamber 110 is designed for a relatively high temperature of the workingmedium M, i.e. about 1000° C. to 1600° C. In order to permit acomparatively long operating time even under these operating parameterswhich are unfavorable for the materials, the combustion chamber wall 153is provided with an inner lining formed by heat shield elements 155 onits side facing the working medium M. Each heat shield element 155 madeof an alloy is equipped with a particularly heat-resistant protectivelayer (MCrAlX layer and/or ceramic coating) on the working medium side,or is made of refractory material (solid ceramic blocks). Theseprotective layers may be similar to the turbine blades, i.e. for exampleMCrAlX means: M is at least one element from the group ion (Fe), cobalt(Co), nickel (Ni), X is an active element and stands for yttrium (Y)and/or silicon and/or at least one rare earth element, or hafnium (Hf).Such alloys are known from EP 0 486 489 B1, EP 0 786 017 B1, EP 0412 397B1 or EP 1 306 454 A1.

Refurbishment means that heat shield elements 155 may need to haveprotective layers taken off (for example by sandblasting) after theiruse. The corrosion and/or oxidation layers or products are then removed.Optionally, cracks in the heat shield element 155 are also repaired. Theheat shield elements 155 are then recoated and the heat shield elements155 are used again.

Owing to the high temperatures inside the combustion chamber 110, acooling system may also be provided for the heat shield elements 155 orfor their retaining elements. The heat shield elements 155 are thenhollow, for example, and optionally also have film cooling holes (notshown) opening into the combustion chamber space 154.

FIG. 5 shows a gas turbine 100 by way of example in a partiallongitudinal section. The gas turbine 100 internally comprises a rotor103, which will also be referred to as the turbine rotor, mounted so asto rotate about a rotation axis 102 and having a shaft 101. Successivelyalong the rotor 103, there are an intake manifold 104, a compressor 105,an e.g. toroidal combustion chamber 110, in particular a ring combustionchamber, having a plurality of burners 107 arranged coaxially, a turbine108 and the exhaust manifold 109. The ring combustion chamber 110communicates with an e.g. annular hot gas channel 111. There, forexample, four successively connected turbine stages 112 form the turbine108. Each turbine stage 112 is formed for example by two blade rings. Asseen in the flow direction of a working medium 113, a guide vane row 115is followed in the hot gas channel 111 by a row 125 formed by rotorblades 120.

The guide vanes 130 are fastened on an inner housing 138 of a stator 143while the rotor blades 120 of a row 125 are fastened on the rotor 103,for example by means of a turbine disk 133. Coupled to the rotor 103,there is a generator or a work engine (not shown).

During operation of the gas turbine 100, air 135 is taken in andcompressed by the compressor 105 through the intake manifold 104. Thecompressed air provided at the turbine-side end of the compressor 105 isdelivered to the burners 107 and mixed there with a fuel. The mixture isthen burnt to form the working medium 113 in the combustion chamber 110.From there, the working medium 113 flows along the hot gas channel 111past the guide vanes 130 and the rotor blades 120. At the rotor blades120, the working medium 113 expands by imparting momentum, so that therotor blades 120 drive the rotor 103 and the work engine coupled to it.

During operation of the gas turbine 100, the components exposed to thehot working medium 113 experience thermal loads. Apart from the heatshield elements lining the ring combustion chamber 110, the guide vanes130 and rotor blades 120 of the first turbine stage 112, as seen in theflow direction of the working medium 113, are heated the most. In orderto withstand the temperatures prevailing there, they may be cooled bymeans of a coolant. Substrates of the components may likewise comprise adirectional structure, i.e. they are monocrystalline (SX structure) orcomprise only longitudinally directed grains (DS structure). Iron-,nickel- or cobalt-based superalloys are for example used as material forthe components, in particular for the turbine blades 120, 130 andcomponents of the combustion chamber 110. Such superalloys are known forexample from EP 1 204 776 B1, EP 1 306 454, EP 1 319 729 A1, WO 99/67435or WO 00/44949.

The guide vanes 130 comprise a guide vane root (not shown here) facingthe inner housing 138 of the turbine 108, and a guide vane head lyingopposite the guide vane root. The guide vane head faces the rotor 103and is fixed on a fastening ring 140 of the stator 143.

1. A layer system, comprising: a substrate; and a ceramic outer ceramiclayer produced from a ceramic powder, comprising: a pyrochlore phase ofthe empirical formula A_(x)B_(y)O_(z) with x, y≈2, z≈7; and a secondaryoxide C_(r)O_(s) with r, s>0.
 2. The layer system as claimed in claim 1,further comprising a metallic bonding layer applied to the substrate,the metallic bonding layer comprising an NiCoCrAlX alloy.
 3. The layersystem as claimed in claim 2, further comprising an inner ceramic layerarranged on the metallic bonding layer, wherein the inner ceramic layercomprises a stabilized zirconium oxide layer.
 4. The layer system asclaimed in claim 3, wherein the stabilized zirconium oxide layer is anyttrium-stabilized zirconium oxide layer.
 5. The layer system as claimedin claim 3, wherein the inner ceramic layer has a layer thickness ofbetween 10% and 50% of a total layer thickness of the inner ceramiclayer plus the outer ceramic layer.
 6. The layer system as claimed inclaim 5, wherein the inner ceramic layer has a layer thickness ofbetween 10% and 40% of the total layer thickness of the inner ceramiclayer plus the outer ceramic layer.
 7. The layer system as claimed inclaim 6, wherein the inner ceramic layer has a layer thickness ofbetween 10% and 30% of the total layer thickness of the inner ceramiclayer plus the outer ceramic layer.
 8. The layer system as claimed inclaim 5, wherein the inner ceramic layer has a layer thickness ofbetween 20% and 40% of the total layer thickness of the inner ceramiclayer plus the outer ceramic layer.
 9. The layer system as claimed inclaim 3, wherein the inner ceramic layer has a layer thickness ofbetween 50% and 90% of a total layer thickness of the inner ceramiclayer plus the outer ceramic layer.
 10. The layer system as claimed inclaim 9, wherein the inner ceramic layer has a layer thickness ofbetween 60% and 90% of the total layer thickness of the inner ceramiclayer plus the outer ceramic layer.
 11. The layer system as claimed inclaim 10, wherein the inner ceramic layer has a layer thickness ofbetween 70% and 90% of the total layer thickness of the inner ceramiclayer plus the outer ceramic layer.
 12. The layer system as claimed inclaim 9, wherein the inner ceramic layer has a layer thickness ofbetween 60% and 80% of the total layer thickness of the inner ceramiclayer plus the outer ceramic layer.
 13. The layer system as claimed inclaim 3, wherein the inner ceramic layer has a layer thickness of from100 μm to 200 μm.
 14. The layer system as claimed in claim 2, whereinthe metallic bonding layer comprises: 11 wt %-13 wt % cobalt, 20 wt %-22wt % chromium, 10.5 wt %-11.5 wt % aluminum, 0.3 wt %-0.5 wt % yttrium,1.5 wt %-2.5 wt % rhenium and Nickel.
 15. The layer system as claimed inclaim 2, wherein the metallic bonding layer comprises: 24 wt %-26 wt %cobalt, 16 wt %-18 wt % chromium, 9.5 wt %-11 wt % aluminum, 0.3 wt%-0.5 wt % yttrium, 1 wt %-1.8 wt % rhenium and nickel.
 16. The layersystem as claimed in claim 3, wherein a total layer thickness of theinner ceramic layer plus the outer ceramic layer is at least 300 μm. 17.The layer system as claimed in claim 16, wherein the total layerthickness of the inner ceramic layer plus the outer ceramic layer 300μm.
 18. The layer system as claimed in claim 3, wherein a total layerthickness of the inner ceramic layer plus the outer ceramic layer is atleast 400 μm.
 19. The layer system as claimed in claim 18, wherein thetotal layer thickness of the inner ceramic layer plus the outer ceramiclayer 400 μm.
 20. The layer system as claimed in claim 1, furthercomprising a metallic bonding layer applied to the substrate, themetallic bonding layer consisting of an NiCoCrAlX alloy.
 21. A layersystem, consisting of: a substrate; an inner ceramic layer; and aceramic outer ceramic layer produced from a ceramic powder, comprising:a pyrochlore phase of the empirical formula A_(x)B_(y)O_(z) with x, y≈2,z≈7; and a secondary oxide C_(r)O_(s) with r, s>0.
 22. A layer system,consisting of: a substrate; a metallic bonding layer arrange on thesubstrate; an oxide layer on the metallic bonding layer; an innerceramic layer; and a ceramic outer ceramic layer produced from a ceramicpowder, comprising: a pyrochlore phase of the empirical formulaA_(x)B_(y)O_(z) with x, y≈2, z≈7; and a secondary oxide C_(r)O_(s) withr, s>0.